Signaling networks are crucial for the orchestration of cellular functions in response to stimuli. Knowledge of the structure of these networks provides a basis for understanding the pathological consequences of their malfunction and offers opportunities for designing therapeutic interventions. The complexity of these networks and the speed with which signals are transmitted in cells makes mapping them a formidable challenge. The typical approach for elucidating the structure of cellular signaling networks involves an iterative process of creating signaling protein disruptions, domain mutants and site-directed mutants followed by characterization of each mutant through a battery of cellular activation assays. As a complementary approach, modern proteomic methods using quantitative mass spectrometry can facilitate the hypothesis-driven characterization of signaling pathways by providing a global view of cellular phosphorylation and protein-protein interactions through a variety of activation states. T cells play a central role in cell-mediated immunity against viruses, a variety of microbes, and cancer. This proposal focuses on the elucidation of the molecular details of the T cell signaling pathway using these new technologies. Lck tyrosine kinase is the central regulator of T cell activation regulated through its phosphorylation state. Lck autophosphorylation at Tyr394 activates the kinase, whereas phosphorylation at Tyr505 inactivates the kinase. Four phosphatases were shown previously to act on Lck Tyr394, but how each one is recruited to Lck and whether other negative regulatory molecules are involved is not understood. The molecular mechanism controlling the proper distribution of Lck between the T cell receptor and downstream signaling nodes such as the SLP76 scaffolded signalosome are not well defined. In the previous funding period, our research team discovered that downstream members of the T cell signaling pathway regulate the phosphorylation of Lck and its substrates. We discovered that the scaffold protein SLP-76 controls both negative and positive feedback loops in T cell receptor signaling at Lck Tyr394. We also discovered that PLC?1 regulates differential Lck substrate phosphorylation within the TCR and the SLP-76 complex. To gain new insights into the pathways regulating Lck activity and spatial localization, we have assembled a multidisciplinary team to apply novel quantitative proteomic techniques, biochemical methods, and mouse models to provide a detailed view of the network. The central question that we will address in this project is how SLP76 and PLC?1 set the spatial and temporal equilibrium of Lck activation resulting in appropriate T cell response to antigen. Successful completion of the aims will clarify the identity of the regulatory proteins employed in each feedback loop, define molecular factors controlling the cellular localization of Lck, and define their physiological role.